For example, in an image forming apparatus using an electrophotographic system, such as a facsimile, a printer or a copying machine, a toner is adhered to an electrostatic latent image formed on the surface of a photosensitive member to visualize it, the visualized image is transferred to a sheet or the like and thereafter it is fixed by being heated and pressurized. In terms of achieving high image quality and colorization, as a developer, a so-called two-component developer containing a carrier and a toner is widely used.
In a development system using a two-component developer, a carrier and a toner are agitated and mixed within a development device, and the toner is charged by friction so as to have a predetermined amount. Then, the developer is supplied to a rotating development roller, a magnetic brush is formed on the development roller and the toner is electrically moved to the photosensitive member through the magnetic brush to visualize the electrostatic latent image on the photosensitive member. The carrier after the movement of the toner is left on the development roller, and is mixed again with the toner within the development device. Hence, as the properties of the carrier, a magnetic property for forming the magnetic brush, a charging property for providing a desired charge to the toner and durability for repeated use are required.
Hence, carriers in which the surfaces of magnetic particles such as magnetite and various types of ferrite are coated with a resin are generally used. A satisfactory magnetic property is required for magnetic particles serving as a carrier core material, and moreover, a satisfactory friction charging property is required for a toner. As the carrier core material which satisfies such a property, carrier core materials having various shapes are proposed (see for example, patent documents 1 and 2).
For example, patent document 1 proposes an electrostatic charge development carrier which contains at least magnetic particles and in which the average interval Sm of projections and recesses in the surface of the magnetic particles and the maximum height Ry of the surface of the magnetic particles satisfy 3.6 μm≦Sm≦8.0 μm and 0.5 μm≦Ry≦1.0 μm.
Patent document 2 proposes an electrostatic latent image development carrier that includes magnetic particles which have projection and recesses in the surface thereof, a conductive layer which is provided in the surface of the magnetic particles, which is formed of conductive metal nanoparticles and which has, in the surface thereof, projections and recesses along the projections and recesses in the surface of the magnetic particles and a resin layer which is provided on the conductive layer. Patent document 2 also discloses magnetic particles whose surface roughness Ra is equal to or more than 0.1 μm but equal to or less than 10 μm and whose surface roughness Sm is equal to or more than 0.1 μm but equal to or less than 10 μm.